During muscular contraction which of the following events occur ?

(a) ‘H’ zone disappears

(b) ‘A’ band widens

(c) ‘I’ band reduces in width

(d) Myosine hydrolyzes ATP, releasing the ADP and Pi

(e) Z-lines attached to actins are pulled inwards

Choose the correct answer from the options given below.

The correct answer is (a), (c), (d), (e) only.

To understand why these events occur during muscular contraction, let us analyze the structural and biochemical changes described by the sliding filament theory of muscle contraction:

1. Structural Components of the Sarcomere:
A sarcomere is the basic functional unit of skeletal muscle, located between two successive Z-lines. It contains:
- 'A' band (Anisotropic band): The dark region containing thick myosin filaments, which overlap with thin actin filaments at their outer margins.
- 'I' band (Isotropic band): The light region containing only thin actin filaments.
- 'H' zone: The central part of the 'A' band that is not overlapped by thin filaments, consisting only of myosin.
- Z-lines: Anchoring structures at the ends of the sarcomere to which actin filaments are attached.

2. Physical Changes During Contraction:
When a muscle is stimulated to contract, the thin actin filaments slide over the thick myosin filaments towards the center of the sarcomere:
- (a) 'H' zone disappears: As actin filaments slide inwards towards the center of the sarcomere, the gap between their opposing ends (the H-zone) decreases and eventually disappears during maximum contraction. Thus, statement (a) is correct.
- (b) 'A' band widens: The 'A' band corresponds to the length of the myosin filaments. Because these filaments do not change their length, the width of the 'A' band remains constant. Thus, statement (b) is incorrect.
- (c) 'I' band reduces in width: As actin filaments are pulled further into the 'A' band, the region containing only actin (the I-band) narrows. Thus, statement (c) is correct.
- (e) Z-lines are pulled inwards: Because the actin filaments are anchored to the Z-lines, sliding them inwards pulls the Z-lines closer together, shortening the overall sarcomere. Thus, statement (e) is correct.

3. Chemical Cycles (ATP Hydrolysis):
- (d) Myosin hydrolyzes ATP, releasing ADP and Pi: The globular head of the myosin molecule functions as an ATPase enzyme. It binds and hydrolyzes ATP to ADP and inorganic phosphate (Pi) to enter an energized state and form a cross-bridge with actin. As the myosin head performs the power stroke to pull the actin filament, it releases the ADP and Pi. Thus, statement (d) is correct.

Evaluating all the statements, we find that (a), (c), (d), and (e) are correct, while (b) is incorrect.